Behavior-shaping constraint

A behavior-shaping constraint, also sometimes referred to as a forcing function or poka-yoke, is a technique used in error-tolerant design to prevent the user from making common errors or mistakes. One example is the reverse lockout on the transmission of a moving automobile.

The microwave provides another example of a forcing function. In all modern microwaves, it is impossible to start the microwave while the door is still open. Likewise, the microwave will shut off automatically if the door is opened by the user. By forcing the user to close the microwave door while it is in use, it becomes impossible for the user to err by leaving the door open. Forcing functions are very effective in safety critical situations such as this, but can cause confusion in more complex systems that do not inform the user of the error that has been made.

When automobiles first started shipping with on-board GPS systems, it was not uncommon to use a forcing function which prevented the user from interacting with the GPS (such as entering in a destination) while the car was in motion. This ensures that the driver's attention is not distracted by the GPS. However, many drivers found this feature irksome, and the forcing function has largely been abandoned. This reinforces the idea that forcing functions are not always the best approach to shaping behavior.